1. Field of the Invention
The present invention relates to liquid crystal displays used in television receivers and display sections of electronic apparatus and, more particularly, to a liquid crystal display in which a polymeric material included in a liquid crystal material is polymerized to impart a pre-tilt angle to the liquid crystal material.
2. Description of the Related Art
There is a recent trend toward larger display screens in the field of liquid crystal displays having a liquid crystal display panel for the use of such displays as display sections of television receivers. For this reason, higher display quality is required for liquid crystal displays. However, it is difficult to achieve characteristics required for a display section of a television receiver using a liquid crystal display employing the TN (Twisted Nematic) method which has been the main stream of the field because of the narrow viewing angle resulting from the method. Under the circumstance, techniques other than the TN method are currently being put in use in order to achieve the property of a wide viewing angle. One of such techniques is referred to as MVA (Multi-domain Vertical Alignment) method. In an MVA type liquid crystal display, liquid crystal molecules in a liquid crystal layer sealed between two substrates combined in a face-to-face relationship are aligned perpendicular to the substrates, and the alignment of the liquid crystal molecules is regulated by protrusions formed on the substrates or slits, provided on a transparent electrode (ITO).
It is known in general that when the vertical alignment method in which liquid crystal molecules are aligned perpendicular to substrates, optical characteristics measured in a direction oblique to a direction normal to the display screen are different from optical characteristics in the normal direction. FIG. 11 is a graph showing characteristics of luminance relative to input gradations (gradation/luminance characteristics) of a vertical alignment type liquid crystal display. The abscissa axis represents input gradations (in gray scale), and the ordinate axis represents luminance (T/Twhite) normalized with reference to the luminance of display of white (TWhite). The curve in a solid line in the figure indicates gradation/luminance characteristics in a direction perpendicular to the display screen (hereinafter referred to as a square direction), and the curve connecting black triangular symbols in the figure indicates gradation/luminance characteristics in a direction at an azimuth angle of 90° and a polar angle of 60° to the display screen (hereinafter referred to as an oblique direction). An azimuth angle is an angle measured counterclockwise with reference to the direction to the right of the display screen. A polar angle is an angle to a line that is vertical to the center of the display screen.
As shown in FIG. 11, gradation/luminance characteristics in a direction oblique to the direction of a polarization axis significantly deviate from gradation/luminance characteristic in the square direction. For example, luminance in the oblique direction is higher than luminance in the square direction in the range of gradations from 0 to 210, whereas luminance in the oblique direction is lower than luminance in the square direction in the range of gradations from 210 to 255 or higher. As a result, when the screen is viewed in the oblique direction, there are small differences in luminance between input gradations, and the color of an image appears more whitish compared to a view of the same in the square direction.
A known solution to this problem is a liquid crystal display having a pixel structure including a pixel electrode electrically connected to a source electrode of a thin film transistor (TFT) for a pixel and another pixel electrode that is separated from the pixel electrode and insulated from the source electrode. In such a liquid crystal display, an electrostatic capacitance is formed by the pixel electrode insulated from the source electrode, the source electrode, and an insulation film sandwiched between the two electrodes. The pixel electrode insulated from the source electrode is driven by the electrostatic capacitance.
FIG. 12 shows a configuration of one pixel of a liquid crystal display having the pixel structure including two separated pixel electrodes. As shown in FIG. 12, a gate bus line 106 and a plurality of drain bus lines 108 are formed on a glass substrate 103, the drain bus lines extending across the gate bus line 106 with an insulation film (not shown) interposed between them. A TFT 110 is disposed in the vicinity of an intersection between the gate bus line 106 and a drain bus line 108, a TFT being formed at each pixel. A part of the gate bus line 106 serves as a gate electrode 110c of the TFT 110. An active semiconductor layer and a channel protection film (both of which are not shown) of the TFT 110 are formed above the gate bus line 106 with an insulation film interposed. A drain electrode 110a along with an n-type impurity semiconductor layer (not shown) underlying the same and a source electrode 110b along with an n-type impurity semiconductor layer (not shown) underlying the same are formed on the channel protection film of the TFT 110 above the gate electrode 110c, the electrodes facing each other across a predetermined gap.
A storage capacitor bus line 114 is formed to extend in parallel with the gate bus line 106 across a pixel region which is defined by the gate bus line 106 and the drain bus lines 108. A storage capacitor electrode (intermediate electrode) 116 is formed at each pixel above the storage capacitor bus line 114 with an insulation film interposed between them. The storage capacitor electrode 116 is electrically connected to the source electrode 110b of the TFT 110 through a connection electrode 111. A storage capacitor Cs is formed by the storage capacitor bus line 114, the storage capacitor electrode 116, and the insulation film sandwiched between them.
The pixel region defined by the gate bus line 106 and the drain bus lines 108 is divided into a sub-pixel 120 and a sub-pixel 122. For example, the sub-pixel 120, which has a trapezoidal shape, is disposed on the left side of a central part of the pixel region, and the sub-pixel 122 is disposed in upper part and lower parts of the pixel region and on the right side of the central part excluding the area of the sub-pixel 120. Referring to the disposition of the sub-pixels 120 and 122 in the pixel region, they are substantially line symmetric about the storage capacitor bus line 114. A pixel electrode 121 is formed at the sub-pixel 120, and a pixel electrode 123, which is separate from the pixel electrode 121, is formed at the sub-pixel 122. Both of the pixel electrodes 121 and 123 are constituted by a transparent conductive film such as an ITO. The pixel electrode 121 is electrically connected to the storage capacitor electrode 116 and the source electrode 110b of the TFT 110 through a contact hole 118 which is an opening in a protective film (not shown). The pixel electrode 123 has a region which overlaps the connection electrode 111 with a protective film and an insulation film interposed between them. In that region, an electrostatic capacitance Cc is formed by the connection electrode 111, the pixel electrode 123, and the protective film sandwiched between the electrodes 111 and 123.
A common electrode, which is not shown, is formed on an opposite glass substrate (not shown) provided opposite to the glass substrate 103. A linear protrusion 112a as an alignment regulating structure for regulating the direction of alignment of the liquid crystal is formed so as to protrude from the opposite glass substrate in a position opposite to the connecting electrode 111 diagonally extending in the figure. A linear protrusion 112b is formed so as to protrude from the opposite glass substrate in a position in which it is substantially line symmetric with the liner protrusion 112a about the storage capacitor bus line 114. Further, a V-shaped linear protrusion 112c is formed such that it is disposed above the pixel electrode 121 on the left side of the central part of the pixel region. The linear protrusion 112c is substantially line symmetric about the storage capacitor bus line 114.
At the sub-pixel 120, a liquid crystal capacitance Clc1 is formed by pixel electrode 121, the common electrode, and the liquid crystal sandwiched between those electrodes. At the sub-pixel 122, a liquid crystal capacitance Clc2 is formed by the pixel electrode 123, the common electrode, and the liquid crystal sandwiched between those electrodes. The liquid crystal capacitance Clc2 and the electrostatic capacitance Cc are connected in series between the glass substrate 103 and the opposite glass substrate.
When the TFT 110 is turned on, the source electrode 110b and the connection electrode 111 bear the same potential as a gradation voltage VD applied to a drain bus line 108, and the pixel electrode 121 in electrical connection with them also bears the same potential as the gradation voltage VD. A voltage originating from a potential difference applied between the pixel electrode 121 and the common electrode is applied to the liquid crystal capacitance Clc1. For example, when the voltage applied to the common electrode is 0 V, the voltage applied to the liquid crystal capacitance Clc1 is equal to the gradation voltage VD (=VD−0V). On the other hand, the pixel electrode 123, which is electrically insulated, is applied with a voltage that is obtained by dividing the gradation voltage VD based on the ratio between the liquid crystal capacitance Clc2 and the electrostatic capacitance Cc. The voltage applied to the pixel electrode 123 (represented by V1) can be expressed as follows.V1=VD×{Cc/(Clc2+Cc)}  (1)
As apparent from the above, there is a difference between thresholds of the pixel electrode 121 which is electrically connected to the source electrode 110b and the pixel electrode 123 which is insulated from the same. Consequently, gradation/luminance characteristics in an oblique direction are significantly improved. As shown in FIG. 11, the curve representing gradation/luminance characteristics in a square direction bulges downward. On the contrary, the curve indicating gradation/luminance characteristics in an oblique direction of an MVA type display in the related art is a mixture of a range in which the curve greatly bulges upward (the range of gradations from 0 to about 210) and a range in which the curve bulges downward (the range of gradations from about 210 to 255). Therefore, missing or spreading gradations can be generated depending on gradation data to be displayed, which results in variation of the color of an image. In the case of a liquid crystal display having the pixel structure shown in FIG. 12, a curve indicating gradation/luminance characteristics of the apparatus in a direction oblique thereto will include substantially no upward or downward bulge, and the apparatus will have significantly high gradation characteristics.
Patent Document 1: JP-A-2003-149647
A liquid crystal display having the pixel structure shown in FIG. 12 can provide improved gradation/luminance characteristics in an oblique direction. However, as indicated by Expression 1, the voltage V1 applied to the liquid crystal capacitance Clc2 of the sub-pixel 122 decreases below the gradation voltage VD. Therefore, the absolute value of the luminance in an oblique direction of the liquid crystal display is smaller than that of a liquid crystal display without such a pixel structure. Further, since a pixel region of the liquid crystal display is divided into two regions, the disposition of the linear protrusions (bank-like structures) and slits in the pixel electrodes (gaps in the pixel electrodes 121 and 123) become complicated. A problem consequently arises in that the aperture ratio is substantially reduced to reduce luminance.